Method and system for wireless communication networks using relaying

ABSTRACT

The present invention relates to wireless networks using relaying. In the method according to the present invention of performing communication in a two-hop wireless communication network, a transmitter  210,  a receiver  220  and at least one relay station  215  are engaged in a communication session. The relay station  215  forwards signals from a first link between the transmitter  210  and the relay station  215  to a second link between the relay stations  215  and the receiver  220.  The forwarding performed by the at least one relay station  215  is adapted as a response to estimated radio channel characteristics of at least the first link. Preferably the forwarding is adapted as a response to estimated radio channel characteristics of both the first and second link.

FIELD OF THE INVENTION

The present invention relates to relay supported wireless communicationto enhance communication performance. In particular the inventionrelates to a method and a system for performing communication in atwo-hop wireless communication network.

BACKGROUND OF THE INVENTION

A main striving force in the development of wireless/cellularcommunication networks and systems is to provide, apart from many otheraspects, increased coverage or support of higher data rate, or acombination of both. At the same time, the cost aspect of building andmaintaining the system is of great importance and is expected to becomeeven more so in the future. As data rates and/or communication distancesare increased, the problem of increased battery consumption is anotherarea of concern.

Until recently the main topology of wireless networks has been fairlyunchanged, including the three existing generations of cellularnetworks. The topology characterized by the cellular architecture withthe fixed radio base stations and the mobile stations as thetransmitting and receiving entities in the networks, wherein acommunication typically only involves these two entities. An alternativeapproach to networks are exemplified by the well-known multihopnetworks, wherein typically, in a wireless scenario, a communicationinvolves a plurality of transmitting and receiving entities in arelaying configuration. Such systems offer possibilities ofsignificantly reduced path loss between communication (relay) entities,which may benefit the end-to-end (ETE) users.

Attention has recently been given to another type of topology that hasmany features and advantages in common with the multihop networks but islimited to only two (or a few) hop relaying. In contrast to multihopnetworks, aforementioned topology exploits aspects of parallelism andalso adopts themes from advanced antenna systems. These networks,utilizing the new type of topology, have cooperation among multiplestations as a common denominator. In recent research literature, it goesunder several names, such as cooperative relaying, cooperativediversity, cooperative coding, virtual antenna arrays, etc. In thepresent application the terms “cooperative relaying” and “cooperativeschemes/methods” is meant to encompass all systems and networksutilizing cooperation among multiple stations and the schemes/methodsused in these systems, respectively. A comprehensive overview ofcooperative communication schemes are given in [1]. Various formats of arelayed signal may be deployed. A signal may be decoded, re-modulatedand forwarded, or alternatively simply amplified and forwarded. Theformer is known as decode-and-forward or regenerative relaying, whereasthe latter is known as amplify-and-forward, or non-regenerativerelaying. Both regenerative and non-regenerative relaying is well known,e.g. by traditional multihopping and repeater solutions respectively.Various aspects of the two approaches are addressed in [2].

The general benefits of cooperative relaying in wireless communicationcan be summarized as higher data rates, reduced outage (due to differentforms of diversity), increased battery life, extended coverage (e.g. forcellular).

Various schemes and topologies utilizing cooperative relaying has beensuggested, as theoretical models within the area of information theory,as suggestions for actual networks and in a few cases as laboratory testsystems, for example. Examples are found in [1] pages 37-39, 41-44. Thevarious cooperation schemes may be divided based on which entities havedata to send, to whom and who cooperates. In FIGS. 1 a-f (prior art)different topologies are schematically illustrated, showing wheretraffic is generated, who is the receiver and the path for radiotransmissions.

The classical relay channel, illustrated in FIG. 1 a, consists of asource that wishes to communicate with a destination through the use ofrelays. The relay receives the signal transmitted by the source througha noisy channel, processes it and forwards it to the destination. Thedestination observes a superposition of the source and the relaytransmission. The relay does not have any information to send; hence thegoal of the relay is to maximize the total rate of information flow fromthe source to the destination. The classical relay channel has beenstudied in [1], [7] and in [3] where receiver diversity was incorporatedin the latter. The classical relay channel, in its three-station form,does not exploit multiple relay stations at all, and hence does notprovide the advantages stated above.

A more promising approach, parallel relay channel, is schematicallyillustrated in FIG. 1 b, wherein a wireless systems employing repeaters(such as cellular basestation with supporting repeaters) withoverlapping coverage, a receiver may benefit of using super-positionedsignals received from multiple repeaters. This is something that happensautomatically in systems when repeaters are located closely. Recently,information theoretical studies have addressed this case. A particularcase of interest is by Schein, [4] and [5]. Schein has performedinformation theoretical study on a cooperation-oriented network withfour nodes, i.e. with one transmitter, one receiver and only twointermediately relays. A real valued channel with propagation loss equalto one is investigated. Each relay employs non-regenerative relaying,i.e. pure amplification. Thanks to the simplistic assumption of realvalued propagation loss, the signals add coherently at the receiverantenna. Under individual relay power constraints, Schein also indicatesthat amplification factors can be selected to maximize receiver SNR,though does not derive the explicit expression for the amplificationfactors. One of the stations sends with its maximum power, whereas theother sends with some other but smaller power. The shortcoming ofSchein's schemes is that it is; only an information theoreticalanalysis, limited to only two relay stations, derived in a real valuedchannel with gain one (hence neglecting fundamental and realisticpropagation assumptions), lacks the means and mechanisms to make themethod practically feasible. For example, protocols, power control andRRM mechanisms, complexity and overhead issues are not addressed at all.With respect to only addressing only two relay stations, thesignificantly higher antenna gains and diversity benefits, as wouldresult for larger number of relays, are neither considered norexploited.

The concept of Multiple-access Channel with Relaying (a.k.a. as Multipleaccess channels with generalized feedback) has been investigated byseveral researchers lately and is schematically illustrated in FIG. 1 c.The concept involves that two users cooperate, i.e. Exchange theinformation each wants to transmit, and subsequently each user sends notjust its own information but also the other users information to onereceiver. The benefit in doing so is that cooperation provides diversitygain. There are essentially two schemes that have been investigated;cooperative diversity and coded cooperative diversity. Studies arereported in [1], for example. With respect to diversity, various formshas bee suggested, such as Alamouti diversity, receiver diversity,coherent combining based diversity. Typically the investigated schemesand topologies rely on decoding data prior to transmission. This furthermeans that stations has to be closely located to cooperate, andtherefore exclude cooperation with more distant relays, as well as thelarge number of potential relays if a large scale group could be formed.An additional shortcoming of those schemes is that is fairly unlikelyhaving closely located and concurrently transmitting stations. Theseshortcomings indicates that the investigated topology are of lesspractical interest. The broadcast channel with relaying, illustrated inFIG. 1 d, is essentially the reverse of the topology depicted in FIG. 1c, and therefore shares the same severe shortcomings.

A further extension of the topology depicted in FIG. 1 c is theso-called interference channel with relaying, which is illustrated inFIG. 1 e, wherein two receivers are considered. This has e.g. beenstudied in [8] and [1] but without cooperation between the receivers,and hence not exploiting the possibilities possibly afforded bycooperative relaying.

Another reported topology, schematically illustrated in FIG. 1 f, issometimes referred to as Virtual Antenna Array Channel, and described infor example [9]. In this concept, (significant) bandwidth expansionbetween a communicating station and adjacent relay nodes is assumed, andhence non-interfering signals can be transferred over orthogonalresources that allows for phase and amplitude information to beretained. With this architecture, MIMO (Multiple Input Multiple Output)communication (but also other space-time coding methods) is enabled witha single antenna receiver. The topology may equivalently be used fortransmission. A general assumption is that relay stations are close tothe receiver (or transmitter). This limits the probability to find arelay as well as the total number of possible relays that may be used. Asignificant practical limitation is that very large bandwidth expansionis needed to relay signals over non-interfering channels to the receiverfor processing.

Cooperative relaying has some superficial similarities to the Transmitdiversity concept in (a.k.a. Transmit diversity with Rich Feedback,TDRF), as described in [10] and is schematically illustrated in FIG. 1g. Essential to the concept is that a transmitter with fixed locatedantennas, e.g. at a basestation in a cellular system, finds out thechannel parameters (allowing for fading effects and random phase) fromeach antenna element to the receiver antenna and uses this informationto ensure that a (noise free) signal, after weighting and phaseadjustment in the transmitter, is sent and adds coherently at thereceiver antenna thereby maximizing the signal to noise ratio. Whiletransmit diversity, with perfectly known channel and implemented in afixed basestation, provides significant performance benefits, it alsoexist practical limitations in terms of the number of antenna elementsthat can be implemented in one device or at one antenna site. Hence,There is a limit in the degree of performance gain that can be attained.A disadvantage for basestation oriented transmit diversity is also thatlarge objects between transmitter and receiver incur high path loss.

Thus, it is in the art demonstrated that cooperative relaying have greatpotentials in providing high capacity and flexibility, for example.Still, the in the art proposed topologies and methods do not take fulladvantage of the anticipated advantages of a network with cooperativerelaying.

SUMMARY OF THE INVENTION

In the state of the art methods, the quality of the first link, thesecond link or a combination thereof is not considered in adapting anytransmission parameters. This has the consequence that performance maydegrade and resources are inefficiently utilized.

Hence, a significant shortcoming of the above discussed prior art isthat they do not adapt transmit parameters of the relays in response ofthe quality of a link or combination of links (first and second)involved in the forwarding procedure. Whereby, the prior art has notbeen able to fully take advantage of the anticipated advantages of acooperative relaying network.

Obviously an improved method and system for a cooperative relayingnetwork is needed, which consider the quality of the first link, thesecond link or a combination thereof in adapting transmission parametersis needed, to whereby have the ability to better take advantage of theanticipated advantages of a cooperative relaying network.

The object of the invention is to provide a method, a relay station anda system that overcomes the drawbacks of the prior art techniques. Thisis achieved by the method as defined in claim 1, the relay station asdefined in claim 12 and the system as defined in claim 16.

The problem is solved by that the present invention provides a method, arelay station and a system that makes is possible to use estimated radiochannel characteristics of both the first and second link for adaptingthe forwarding of signals from a first link to a second link performedby the relay station.

In the method, according to the present invention of performingcommunication in a two-hop wireless communication network, atransmitter, a receiver and at least one relay station are engaged in acommunication session. The relay station forwards signals from a firstlink between the transmitter and the relay station to a second linkbetween the relay stations and the receiver. The forwarding performed bythe at least one relay station is adapted as a response to estimatedradio channel characteristics of at least the first link. Preferably theforwarding is adapted as a response to estimated radio channelcharacteristics of both the first and second link.

The relay station according to the present invention is adapted for usein a two-hop wireless communication network, wherein the networkcomprises a transmitter, a receiver and at least one relay station. Therelay station is adapted to forward signals from a first link betweenthe transmitter and the relay station to a second link between the relaystations and the receiver. The relay station is provided with means foradapting the forwarding based on characterization of both the first andsecond link.

Thanks to the invention it is possible to better adjust the forwardingon the second link to the actual conditions present during acommunication session. In addition the forwarding can be better adjustedto changes in the conditions.

One advantage afforded by the present invention is that the more preciseand reliable characterization of the individual radio paths may be usedto determine and optimize different transmission parameters. Whereby,the capabilities of a cooperative relaying network, for example, may bemore fully exploited.

A further advantage is that characterisation of the first and secondlink advantageously is performed in the relay stations. Hence, themethod according to the invention fascilitates a distribution offunctionalities in the network allowing an increase in the number ofrelay stations in a communication session without any significantincrease in the amount of protocol overhead that is needed for thetransmission of data from the transmitter to the receiver.

A yet further advantage further advantage of the method and systemaccording to the present invention is that the improved characterizationof the first and second link facilitate to take full advantage of theanticipated advantages of a network with cooperative relaying thatcomprises a larger number of relaying stations. With the invention usedin a coherent combining setting, the directivity gain and diversity gainincreases with increasing number of relay stations. The directivity gainitself offers increased SNR that can be used for range extension and/ordata rate enhancement. The diversity gain, increases the robustness ofthe communication, providing a more uniform communication quality overtime. While directivity and diversity gain can be provided by varioustraditional advanced antenna solutions, where the antennas are placedeither at the transmitter or the receiver, the proposed solution isgenerally not limited to the physical space constraints as is seen inbasestations or mobile terminals. Hence, there is indeed a potential touse a larger number of relays, than the number of antennas at abasestation or a mobile station, and hence offer even greaterdirectivity and diversity gains.

Embodiments of the invention are defined in the dependent claims. Otherobjects, advantages and novel features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE FIGURES

The features and advantages of the present invention outlined above aredescribed more fully below in the detailed description in conjunctionwith the drawings where like reference numerals refer to like elementsthroughout, in which:

FIG. 1 a-g are schematic illustrations of the topologies of some priorart utilizing cooperative relaying;

FIG. 2 schematically illustrates a cellular system using cooperativerelaying according to the present invention;

FIG. 3 is a schematic model used to describe the parameters and termsused in the present invention;

FIG. 4 is a flowchart over the method according to the invention;

FIGS. 5 a and 5 b are a schematic illustrations of two alternativelogical architectures for the cooperative relaying network according tothe present invention;

FIG. 6 is a flowchart over one embodiment of the method according to theinvention;

FIG. 7 is a schematic illustration of an alternative embodiment of theinvention utilizing relay stations with multiple antennas;

FIG. 8 is a schematic illustration of an alternative embodiment of theinvention utilizing direct transmission between the transmitter and thereceiver;

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to thefigures.

The network outlined in FIG. 2 is an example of a cooperative relayingnetwork wherein the present invention advantageously is implemented. Thefigure shows one cell 205 of the wireless network comprising abasestation 210 (BS), a plurality of relay stations 215 (RS) and aplurality of mobile stations (MS) 220-223. As shown in the figure, therelay stations 215 are mounted on masts, but may also be mounted onbuildings, for example. Fixed relays may be used as line of sightconditions can be arranged, directional antennas towards the basestationmay be used in order to improve SNR (Signal-to-Noise Ratio) orinterference suppression and the fixed relay may not be severely limitedin transmit power as the electricity supply network typically may beutilized. However, mobile relays, such as users mobile terminals, mayalso be used, either as a complement to fixed relays or independently.The mobile stations 221 and 222 are examples of mobile relays, i.e.mobile stations that temporarily functions also as relays. The mobilestation 220 is in active communication with the base station 210. Thesignalling, as indicated with arrows, is essentially simultaneouslyusing a plurality of paths, characterized by two hops, i.e. via a relaystation 215 or a mobile station acting as a mobile relay 221, 222. Thetransmission will experience interference from for example adjacentcells, and the effect of the interference will vary over the differentpaths.

It should be noted that although relay based communication is used toenhance communication, direct BS to MS communication may still be used.In fact, some basic low rate signalling between BS and MS may berequired for setting up a relay supported communication channel. Forexample, a cellular system function such as paging may not use coherentcombining based relaying as the relay to MS channels are not a prioriknown, instead preferably, a direct BS to MS communication is usedduring call setup and similar procedures. The real world cellular systemoutlined in FIG. 2 is modeled by system model shown in FIG. 3, here withfocus on a single pair of transmitter and receiver, with an artibtrarynumber K of relay stations. The notation is adapted to a basestation 210as a transmitter and a mobile station 220 as a receiver, but not limitedthereto. The communication between the basestation 210 and the mobilestation 220 can be described as comprising two main parts: thetransmissions from the base station 210 to the relay stations 215:kreferred to as Link 1, and the transmissions from the relay stations215:k to the mobile station 220 referred to as Link 2.

The transmitter, i.e. BS 210 transmits with a power P_(BS). Each relaystation 215:k, wherein kε{1, 2, . . . , K} and K is the total number ofrelay stations, receive the signal and re-transmits with a total powerP_(k). The aggregate transmit power of all relay stations 215:k isdenoted P_(RS). h_(1,k) is the complex path gain from the basestation210 to relay station k 215:k, and h_(2,k) is the complex path gain fromthe relay station k to the mobile station, i.e. h_(1,k) and h_(2,k)characterizes the individual signal paths. The receiver, i.e. MS 220,receives a total signal denoted C_(r) and experience the total noiseN_(r).

Typically, in a realistic scenario a BS in a cell is simultaneouslyengaged in communication with a plurality of mobile stations. This canbe envisaged by considering each communication as modeled in accordanceto FIG. 3. For clarity only a communication session involving one BS,one MS and a plurality of relay station will be considered in thepresent application. However, as will be apparent for the skilled in theart the inventive architecture and method/scheme is easily applied alsoin the case with a plurality of simultaneous communications between thebase station and mobile stations.

As realized by the skilled in the art, in a network according to theabove model, a large number of parameters need to be set and preferablyoptimized in order to fully take advantage of the possibilities andcapacity offered by such a network. This is also, as previouslydiscussed, there the prior art systems display their shortcomings asmulti-relay systems, due to their presumed complexity, are notdiscussed. Parameter that needs to be considered and preferablyoptimized include, but is not limited to, transmit power of thebasestation 210 and each relay station 215:k, which relay stations thatshould be used in the communication, phase control (if coherentcombining is used), coding, delay (in the case of delay diversity),antenna parameters (beamforming, spatial multiplexing), etc. Theparameters needed to control and optimize the transmission will bereferred to as transmission parameters (TP). A preferred optimizationincludes, but is not limited to, optimizing the transmit powers of thebase station 210 and the relay stations 215:k in order to obtain aspecific SNR at the receiving mobile station, which in turn correspondto a certain quality of service or capacity, for example, with regardsto power consumption of the different entities and the interferencelevel in the cell and adjacent cells, for example.

Fundamental to all optimization and necessary for an efficient use ofthe radio recourses is an accurate characterization of the radio pathsin the first and second link, and control over how any changes in anytransmission parameter will affect the overall performance. The methodaccording to the present invention provides a method wherein a relaystation 215:k uses channel characteristics of both the first and secondlink to determine transmission parameters for the forwarding on thesecond link. In addition, according to the method, each relay station215:k may optionally adapt its forwarding on the second link to aquality measure on the communication in full as perceived by thereceiver 220, for example. The quality measure on the communication infull will be referred to as the common transmission parameter.

In the method according to the present invention of performingcommunication in a two-hop wireless communication network, a transmitter210, a receiver 220 and at least one relay station 215 are engaged in acommunication session The relay station 215 forwards signals from afirst link between the transmitter 210 and the relay station 215 to asecond link between the relay stations 215 and the receiver 220. Theforwarding performed by the at least one relay station 215 is adapted asa response to estimated radio channel characteristics of at least thefirst link. Preferably the forwarding is adapted as a response toestimated radio channel characteristics of both the first and secondlink.

The method according to the invention will be described with referenceto the flowchart of FIG. 4 The method comprises the main steps of:

400: Send pilots on the k paths of link 1;

410: Characterize the k paths of link 1.

420: Send pilots on the k paths of link 2;

430: Characterize the k paths of link 2.

440: Determine relative transmission parameters for each relay station215, wherein each relative parameter is based on the characterization ofthe respective paths of link 1 or a combination of link 1 and link 2.

450: Each relay station 215:k adapts the forwarding on link 2 to thereceiver 220 using its respective relative transmission parameter.

Optionally the method comprises the step of:

445: Determining a common transmission parameter reflecting the qualityof the communication in full.

447: Distribute the common transmission parameter to the relay stations(215).

and step 450 is subsequently replaced with:

450′: Each relay station 215:k adapts the forwarding on the second linkto the receiver 220 using its respective relative transmission parameterand the common transmission parameter.

“Pilots” and “sending pilots” should be interpreted as sending any kindof channel estimation symbols. “Hello messages” may also be used forthis purpose.

It should be noted that the sending of pilots does not have to occur inthe above order and may also be simultaneous on link 1 and 2.

The characterization of the radio paths in steps 410 and 430 ispreferably adapted to the transmission technique used, and possibly alsoto the type of optimization which should utilize the characterization.The characterization may comprises of, but is not limited to: estimatingcomplex path gains h_(1,k) and h_(2,k) characterizing each path of thefirst and second link, respectively.

As there are two links, transmitter to relay and relay to receiver,there are four possibilities of which station(s) transmit and whichstation(s) estimate the channel(s). The four possibilities aresummarized in Table 1. The purpose is to illustrate that severaldifferent implementation approaches of the invention may be taken. TABLE1 Link 1 Link 2 Case Transmitter Relay Relay Receiver 1 Send pilotEstimate ch. Estimate ch. Send pilot 2 Send pilot Estimate ch. Sendpilot Estimate ch. 3 Estimate ch. Send pilot Estimate ch. Send pilot 4Estimate ch. Send pilot Send pilot Estimate ch.

Given that channel estimation has been performed in some station, it isalso an issue who perform processing of the collated information, i.e.determine the relative transmission parameters. Essentially, there arethree choices, the transmitter BS 210, the receiver MS 220 or a set ofrelay stations RS 215. Since it is the relay stations that must performthe adjustments of the forwarding on link 2, this is the preferred placeto determine the relative transmission parameters. If a relay stationsends a pilot signal, a representation of the channel characterizationneeds to be reported back to the relay. If a relay station insteadreceives a pilot, the representation of the channel characterizationdoes not need to be reported anywhere (corresponding to case 1). Caseone is in many situations the preferred alternative, since it minimizesthe overhead signalling. On the other hand, one may want to keep therelay stations as simple as possible and perform all calculations in thereceiver and/or transmitter, or in entities in connection with thereceiver or transmitter. If, so case 4 of table 1 may be preferred, andall estimation and calculation is performed in other entities than therelay stations. The information needed for the relay stations to adjusttheir respective forwarding is sent to each relay station. Asillustrated, many possible combinations exist and the invention is notlimited to a specific one.

A preferred system according to the invention, adapted to be able toeffectuate the above-described case 1, will be described with referenceto FIG. 5 a. Each relay station 215:k has means for performing channelcharacterization 216 and means for determining relative transmissionparameters 217 based on the channel characterization and means foradjusting 218 the forwarding based on relative transmission parametersand optionally on a common transmission parameter. The receiver 220 hasmeans for performing a quality measure of the collective signal 221 andoptionally means for determining a common transmission parameter 222.The common transmission parameter is distributed from the receiver 220to the relay stations 215:k either as a direct broadcast to the relaystations 215:k or via the transmitter 210. The relay stations 215:kreceive the common transmission parameter and in combination with theirrelative transmission parameters adjust their forwarding of the signal.This can be seen as comprising a logical control loop between thereceiver 220 and the relay stations 215:k. Typically another logicalcontrol loop exists between the receiver 220 and the transmitter 210,regulating the transmitter's transmission parameters such as outputpower, modulation mode etc. Hence, the preferred embodiment of thepresent invention comprises two logical control loops: a first controlloop 505 between the receiver 220 and the relay stations 215:k,providing the relay stations with the common transmission parameter, anda second control loop 510 feed-backing transmission information from thereceiver 220 to the transmitter 210.

In an alternative embodiment, adapted to be able to effectuate the abovedescribed cases 3-4. and described with reference to FIG. 5 b, the meansfor performing channel characterization 216 and means for determiningboth the relative transmission parameters 217 and the commontransmission parameters 222 is centralized located in the receiver 220,for example. The receiver receives the unprocessed results of the pilotfrom the relay station 215 and/or transmitter 210. The receiver performsthe necessary estimations and sends information on the relativetransmission parameters and the common transmission parameter to therelay stations 215, either as a broadcasted message including allrelative transmission parameters or as dedicated messages to each relaystation. Alternatively may the transmitter perform the estimation of theradio paths of the first link (case 2), and hence, have the meanstherefore. A further alternative is that the characterization and thedetermination of transmission parameters is performed. However,preferably the receiver and transmitter communicate to present acollected message, or messages, with all transmission parameterinformation to the relay stations, either as a broadcasted message toall relay stations or as dedicated messages to each relay station. Afurther alternative is that the characterization and the determinationof transmission parameters is performed elsewhere in the network, forexample in a radio network controller (RNC) or an entity with similarfunctionality.

As described the present invention makes it possible to more precise andreliable determine and optimize different transmission parameters. Thisis turn makes it possible to fully take advantage of the capabilities ofa relaying network, in particular the capabilities of a cooperativerelaying network.

The method according to the invention facilitates a distribution offunctionalities in the network allowing an increase in the number ofrelay stations in a communication session without any significantincrease in the amount of protocol overhead that is needed for thetransmission of data from the transmitter to the receiver.

To efficiently implement the method according to the above, a procedureof taking the characterization of the radio paths of both the links, andpossibly common quality measures, into account in determining theforwarding parameters is desirable. An efficient procedure is outlinedbelow and a full derivation of included expressions “derivation ofanalytic expressions” is given at the end of the detailed description.How the procedure can be adapted to control and optimize transmittedpower, phase and relay station activation, representing differentembodiments, is also given below.

Each relay station k transmits with a total power defined by$\begin{matrix}{P_{k} = \frac{P_{kS} \cdot {a_{k}}^{2}}{\sum\limits_{k = 1}^{K}{a_{k}}^{2}}} & (1)\end{matrix}$, where P_(RS) is the aggregate transmit power of all relay stations,a_(k) is a un-normalized complex gain factor for relay station kε{1, 2,. . . , K} and K is the total number of relay stations.

In “derivation of analytic expressions” it is shown that the maximumreceiver SNR is attained (provided received signal is normalized to unitpower) if $\begin{matrix}{{a_{k}} = \frac{\sqrt{\Gamma_{{RS},k}} \cdot \sqrt{\Gamma_{{MS},k}} \cdot \sqrt{\Gamma_{{RS},k} + 1}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1}} & (2)\end{matrix}$, and ifarg{a _(k)}=−arg{h _(1,k)}−arg{h _(2,k)}  (3)where$\Gamma_{{RS},k} = \frac{{h_{1,k}}^{2}P_{BS}}{\sigma_{{RS},k}^{2}}$, and $\Gamma_{{MS},k} = \frac{{h_{2,k}}^{2}P_{RS}}{\sigma_{MS}^{2}}$, and P_(RS) is the transmit power of the basestation, σ² _(RS,k) is thenoise plus interference level at any relay station, σ² _(MS) is thenoise level at the mobile station, h_(1,k) is complex path gain from thebasestation to relay station k, and finally h_(2,k) is complex path gainfrom the relay station k to the mobile station.

It is can be shown (see the detailed derivation) that a relay station kthat receives a signal y_(k) shall transmit the following signal$\begin{matrix}{z_{k} = {y_{k}{\frac{1}{\sqrt{\sum\limits_{k = 1}^{K}{a_{k}}^{2}}} \cdot \frac{\sqrt{P_{RS} \cdot \Gamma_{{RS},k} \cdot \Gamma_{{MS} \cdot k}}}{\sigma_{{RS},k} \cdot \left( {\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1} \right)} \cdot {\mathbb{e}}^{{- j} \cdot {({{\arg{(h_{1,k})}}1\quad\arg\quad{(h_{2,k})}})}}}}} & (4)\end{matrix}$

It should be noted that Γ_(RS,k) refers to the radio paths of the firstlink and Γ_(MS,k) refers to the radio paths of the second link. Hence,the radio characteristics of both links are taken into account in eachrelay stations forwarding, Γ_(RS,k) and Γ_(MS,k) are preferably, but notnecessarily calculated at each relay station.

The Σ|a_(k)|² term act as a power normalization factor, denoted φ, andit is observed that it cannot be determined individually by each relay.Instead it is hinted here that φ must be determined at some othersuitable station and distributed to the relays. l/φ corresponds to thecommon transmission parameter, and$\frac{\sqrt{P_{RS} \cdot \Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}}{\sigma_{{RS},k}^{2} \cdot \left( {\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1} \right)} \cdot {\mathbb{e}}^{{- j} \cdot {({{\arg{(h_{1,k})}} + {\arg{(h_{2,k})}}})}}$to the relative transmission parameter for relay station k.

The maximum attainable receiver SNR under aggregate relay transmit powerconstraint can be determined to $\begin{matrix}{\Gamma_{Eff}^{(\max)} = {\sum\limits_{k = 1}^{K}\frac{\Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1}}} & (5)\end{matrix}$

At closer inspection, it is noted that the SNR contribution from eachindividual relay to Γ^((max)) _(Eff) is equivalent to that if each relaystation would transmit with all relay transmit power P_(RS) themselves.

Moreover, “derivation of analytic expressions”, expressions for acombination of regenerative and non-regenerative coherent combining isalso presented. When studying regenerative and non-regenerative coherentcombining an interesting observation is that a regenerative approach isgenerally inferior to non-regenerative case, because regenerativerelaying by necessity is constrained to a region around the transmitterand cannot exploit all available relays in an optical manner. With otherwords, even though a signal may not be decoded, it may still contributewhen coherent combining is employed. In any case, a combination ofnon-regenerative and regenerative scheme will perform slightly betterthan if only the non-regenerative method is considered. The mechanismsfor power and phase control that are discussed in the following are isindependent and generic to whether regenerative relaying is employed aswell.

Phase Control

As the first implementation example the logical architecture and themethod according to the present invention is adapted for the use offacilitating coherent combining. A prerequisite for coherent combiningis that signals are phase-aligned at the receiver. This is enabled bycompensating for the complex phase from the transmitter 210 to the relaystation 215 as well as the complex phase form the relay station 215 tothe receiver 220. Practically, in each relay station the receivedsignal, y_(k), is multiplied with the phase factor e^(−j.arg(a) ^(k) ⁾where arg{a_(k)}=−arg{h_(1,k)}−arg{h_(2,k)}.

Therefore, explicit or implicit channel phase information must be madeavailable at each individual relay station. There are essential twobasic schemes that can be used in deriving phase information, one basedon closed loop control and one on open loop control. The closed loopcontrol is necessary to use when channel reciprocity cannot beexploited, such as in FDD (used over a single link), or when highcontrol accuracy is required. The open loop control scheme insteadexploits channel reciprocity, e.g. enabled by TDD (used over a singlelink) with channel sounding that operates within channel coherence time.Open loop control is generally less accurate than closed loop control,due to asymmetries in the transmit/receive chains for a station. Thedifferences boils down to the effort put into hardward design, and canalways be compensated by improved design. Also, incorporating occasionalclosed loop control cycles may compensate for static open loop errors.However, in the present invention the phase error can in principle be upto ±90 degrees and still combine coherently (but not very efficiently)with other relayed signals. Hence, absolute phase accuracy is not amust, but certainly preferred. A closed control scheme generally relieson explicit signalling, reporting the result of measurements andtherefore consumes more communication resources and incurs latencyrelative an open loop scheme. Note that this discussion on TDD vs. FDDconsiders duplexing technique over a single link at a time, e.g. therelay station to receiver link, whereas it is also possible tocharacterize the overall communication in the network on basis of timeand frequency division. For example, link one and link two may share afrequency band or use different ones. From point of view of theinvention, however, any combination of duplexing and multiple accessschemes may be used, as long as channel phase information can bedetermined and used for phase compensation in the relay stations.

Tightly connected with closed loop and open loop control is the issuewhich station sends the pilots, which has been discussed previously inreference to table 1. Since it is the relay stations that must performphase adjustment, this is the natural place to determine arg{a_(k)}. Ifa relay station sends a pilot signal, the phase (or channel) parametersneed to be reported back to the relay. This corresponds to the closedloop case. If a relay station instead receives a pilot, the phase (orchannel) parameter does not need to be reported anywhere. Thiscorresponds to the open loop case. It is clear that depending whetherphase (i.e. channel) information need to be sent away in a controlpacket or can be kept in the same station, this has an impact on radioresource efficiency, power consumption as well implementationcomplexity. In any case, as seen form above, a myriad of possibilitiesexist and we select the most promising. A preferred combination ofduplexing and multiple access will be further discussed. However, asappreciated by the skilled in the art a very large number ofpossibilities exist and the invention is not limited to the belowexemplified.

Case one (see table 1), which is of open loop type and suitable for TDDwith “sufficient” coherence time, offers the lowest signallingcomplexity as only two transmissions are necessary and the processing isdistributed on all relay stations. Here, the transmitter as well as theintended receiver issue channel estimation symbols often enough orwhenever needed such that each relay can track both channels. The relaystation subsequently estimates the channel phases that determine thephase factor of a_(k).

Power Control

A second important aspect for resource efficient communication, apartfrom phase control, is power control, since it provides means to ensuresatisfactory communication quality. The logical architecture and themethod according to the present invention is readily adapted to be usedfor an effective power control. The power control method is based onthat the effective SNR at the receiver is controlled towards a targetSNR, Γ₀, which assert the desired link quality. The target SNR may ofcourse change with time depending on how link mode or QoS requirementchanges with time. According to the logical architecture and the methodaccording to the present invention power may be adjusted at thetransmitter and individually at each relay. The relay power control hascommon as well as individual relay component. In the objective ofminimizing the aggregate power addresses the issue of multiple accessinterference minimizations as well as minimizing relay powerconsumption. However, when a MS act as a transmitter, the power controlmay also be use as a method for significantly minimizing powerconsumption and radiated power for the MS, which among other advantagesprolongs the batter life of the MS.

On the highest level, the power control problem may be defined as:Find{P_(RS), P_(k)},  ∀  k   ∈ {1, 2, …  , K}; such  that  Γ_(eff)^((max )) = Γ₀This is preferably accomplished under some constraints, such asminimization of P_(RS)=ΣP_(k) and with fixed P_(BS), but otherconstraints may also be considered, e.g. minimization of the totaltransmit power P_(RS)+P_(BS) or by taking localization of relay inducedinterference generation into account. In the following, we assumeminimization of P_(RS)=ΣP_(k) with fixed (or relatively slow) adaptationof P_(BS). This is a reasonable design objective in downlink, but foruplink it may be of greater interest to minimize the transmitter power.However, if the relays are mobile and relay on battery power, the sumpower of relays and transmitter may be minimized.

This is the basic function of power control. From practical viewpoint,the overall task of controlling power in a cooperative relay network ingeneral, and with coherent combining in particular, is to use previousknowledge of used power P_(BS) and P_(k) and update those parameters tomeet desired communication quality.

Power control share much of its traits with the phase control as thegain of the links may be estimated in several ways, depending onclose/open loop, TDD/FDD, distribution of control aspects. Hence, alsohere can a range of alternative implementations be envisioned. In thefollowing, similar to the phase control discussion, it is assumed thatthe transmitter and receiver issue channel estimation signals and thatchannel gain reciprocity can be assumed, but the invention is notlimited hereto.

The power control being proposed here has both a distributed componentfor each relay station, the relative transmission parameter, and acomponent common to all relays, the common transmission parameter. Thescheme operates as follows: Through channel estimation, and withknowledge of the power used to send the pilot, each relay station maydetermine its respective path gain towards the transmitter and receiverrespectively, but also interference and noise levels may be estimated atthe same time. Based on path gain measurement, and information aboutP_(RS) and σ² _(MS), it is possible to determine Γ_(MS,k). Possibly alsobased on path gain, noise with interference estimation and P_(BS)awareness, or simply direct SNR measurements on any received signal, theSNR at the relay station, Γ_(RS,k), can be determined. Based on this,the relative transmit power levels can be determined at each relaystation in a fully distributed manner. However, each relative transmitpower level need to be sealed with normalization factor φ to ensure thataggregate transmit power is identical, or at least close, to theaggregate transmit power P_(RS). This is the common power control part.If φ is too small, then more power than optimum P_(RS) is sert, andhence a more optimal relative power allocation exist for the investedtransmit power. The same is valid when φ is too large. Hence, it isimportant for optimal resource investment to control φ such that theintended power P_(RS) is the aggregate transmit power level by therelays. N.B., it is not a significant problem from performance point ofview if φ is somewhat to small as that only improves the effective SNR,since the relative impact of receiver internal noise is reduced.

Referring now to the logical architecture illustrated in FIG. 5 thenormalization factor, being a common transmission parameter, ispreferably determined, as well as distributed from, the receiver. Thisshould be seen as a logical architecture, since it is also possible toforward all control information to the transmitter, which thenredistribute it to the relay stations, fore example. The first controlloop 505 between the receiver 220 and the relay stations 215:k, providesthe relay stations with the P_(RS), whereas the second control loop 510from the receiver 220 to the transmitter 210, provides the transmitterwith P_(BS). Optionally, if the transmitter has a better view of thewhole radio system including many groups of cooperative TS-RS-RX links,similar to what a backbone connected basestation in a cellular systemwould have, then it may incorporate additional aspects that strive tooptimize the whole system.

One method to implement the control loop at the receiver is now given,then assuming that P_(BS) is fixed (or controlled slowly). From atransmission, occurring at time denoted by n, the receiver measure thepower of the coherently combined signal of interest, C_(r), the relayinduced noise measured at the receiver, N_(r), and the internal noise inthe receiver N_(t). Based on this, and conditioned Γ₀, the receiverdetermines P_(RS)^((n + 1))and an update of a normalization factor, φ^((n+1)). This can be writtenas a mapping through an objective function ƒ as $\begin{matrix}{\left. {f\left( {C_{r},N_{r},N_{i}} \right)}\rightarrow\left\{ {P_{RS}^{({n + 1})},\varphi^{({n + 1})}} \right\} \right.;{{{such}\quad{that}\quad\Gamma_{eff}^{(\max)}} = \Gamma_{0}}} & (6)\end{matrix}$The receiver then distributed the updates, P^((n+1)) _(RS) andφ^((n+1)), to all relays through a multicast control message. Toillustrate the idea, assume that P_(RS) is kept fixed from previoustransmission, but the normalization factor is to be adapted. In thesection “Derivation of analytic expression” it is shown that optimumnormalization requires a balance between received signal, C_(r), and thetotal received noise, interference and receiver internal noiseN_(r)+N_(i) according toC _(r)=(N _(r) +N _(i))³   (7)Hence, by including the previous normalization factor φ^((n)), which isknown by the receiver, and the update needed φ^((n+1)) to balance theequation, the relation becomes $\begin{matrix}{{C_{r}\frac{\varphi^{(n)}}{\varphi^{({n + 1})}}} = \left( {{N_{r}\frac{\varphi^{(n)}}{\varphi^{({n + 1})}}} + N_{i}} \right)^{2}} & (8)\end{matrix}$, which yields φ^((n+1)) by solving a simple second order equation.

If both P_(RS) and φ need to be updated, the balance equation above, therelation for the receiver SNR, Γ, can be used together with measuredsignal levels and solve for P_(RS) and φ. Linearization techniques, suchas Taylor expansion and differentials, may preferably be used for thispurpose and solving for ΔP_(RS) and Δφ.

It is noted that for the first transmission, the normalization factor isnot given a priori. Different strategies may be taken to quickly adaptthe power. For instance, an upper transmit power limit may initially bedetermined by each relay as they can be made aware of Γ₀ and also candetermine their (coherent combining) SNR contribution. If each relaystays well below this upper limit with some factor, power can be rampedup successively by the control loop so ongoing communications are notsuddenly interfered with. This allows control loops, for othercommunication stations, to adapt to the new interference sources in adistributed and controlled manner.

Also note that even though transmit power limitations occur in anyrelays, the power control loop ensures that SNR is maximized under allconditions.

Another, possibly more precise, method to determine the normalizationfactor is to determine the |a_(k)| term in each relay and then send itto the receiver where Σ|a_(k)|², is calculated and hence yielding thenormalization factor φ. Subsequently φ is distributed to all relays,similar to previous embodiment. Note that the amount of signalling maybe reduced and kept on an acceptable level by sampling only a subset ofall relays, i.e. some of the most important relays, in order to producea sufficient good estimate of Σ|a_(k)|² cm. This is further motivatedthat the Σ|a_(k)|² term will generally not change much over short time,even in fading channels, due to large diversity gains inherent in theinvention.

Although power control has been described in the context of coherentcombining, the framework is also applicable for power control in othertypes of relay cooperation schemes, such as various relay inducedtransmit diversity, such as Alamouti diversity. The framework is similarin that the power control considers combinations of transmitter power,individual relay power and aggregate relay power. Another example ofrelay induced transmit diversity is (cyclic/linear) delay diversity.Each relay imposes a random or controlled linear (or cyclic) delay onthe relayed signals, and hence causes artificial frequency selectivity.Delay diversity is a well known transmit diversity from CDMA and OFDMbased communication.

To summarize this section, this invention suggests using power controlas a concept to ensure performance optimization for coherent combiningbased cooperative relaying in a realistic channel and in particular tooptimize signal to noise ratio under aggregate relay transmit powerconstraints. This power control concept is not limited to coherentcombining based cooperative relaying networks, but also othercooperative relaying oriented networks may use the same concept, thoughthen with optimization objectives most suitable to the scheme beingused. In addition, the basic features for a protocol based on channelsounding and estimation of gain parameters over both link one and linktwo are suggested. A reasonable design choice for protocol design (withcommonalities with the phase control) has also been outlined, based onlow complexity, low signalling overhead and low total power consumption.In particular, it is shown that combination of power control loopsincluding relay and transmitter power control may be used. Lastly, ithas been demonstrated that the control loop for the relays may be buildon distributed power control decisions in each relay as well as a commonpower control part, where the whole set of relays are jointlycontrolled.

The main steps of the embodiment using the inventive method andarchitecture for efficient power control and phase control areillustrated in the flowchart of FIG. 6. The method comprises the stepsof:

600: Send pilots on the k paths of link 1, from transmitter 210′ torelay stations 215:k;

610: Each relay station 215:k estimates the k channel of link 1,h_(1,k); Also interference and noise levels are estimated in order tocalculate P_(RS,k).

620: Send pilots on the k paths of link 2, from receiver 220′ to relaystations 215:k;

630: Each relay station 215:k estimates its respective channel out ofthe k channel of 2, h_(2,k);

610: Each relay station 215:k determines relative transmissionparameters based on the channel estimates.

650: The receiver 220′ determines a normalization factor φ.

660: The receiver 220′ broadcast the normalization factor φ, P_(RS), andφ² _(RS) to the relay stations 215:k.

670: Each relay station 215:k uses the broadcasted φ, P_(RS), and thelocally determined Γ_(MS,k) and Γ_(RS,k), and the phase of channelestimates h_(1,k), h_(2,k) to, on the reception of signal y_(k),transmit the following signal:$z_{k} = {{y_{k} \cdot \frac{1}{\varphi}}{\frac{\sqrt{P_{RS} \cdot \Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}}{\sigma_{{RS},k} \cdot \left( {\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1} \right)} \cdot {\mathbb{e}}^{{- j} \cdot {({{\arg{(h_{1,k})}} + {\arg{(h_{2,k})}}})}}}}$wherein the parameters Γ_(RS,k) is calculated based on the channelestimate, P_(BS), and σ² _(RS), and Γ_(MS,k) based on P_(RS), and σ²_(MS).

If the first transmission to the receiver is considered, (then the powerloop is unaware of the forthcoming link quality), by way of example therelay may modify and upper limit the received normalization factor φsuch that φ_(k)=c·|a_(k)|², where c≦1 being sent from the receiver or isa prior known.

675: The receiver 220′ feedbacks control information to the transmitter210′ (P_(BS)).

The first control loop, indicated in step 660 may further comprise thesubsteps of:

660:1 The receiver measure at time n, the quality of the receivedsignal, or more specifically the power of the coherently combinedsignal, C_(r), the relay induced noise measured at the receiver, N_(r),and the internal noise in the receiver N₁.

660:2 The receiver determines based on the measurement of step 675:1,and conditioned a desired Γ₀ target, an update of at least one of thenormalization factor, φ^((n+1)) and the aggregate relay powerP_(RS)^((n + 1)).

660:3 The receiver distributed the updates, P_(RS)^((n + 1))and φ^((n+1)), to all relays through a multicast control message.

Similarly, the second control loop, indicated in step 675, mayoptionally comprise:

675:1 The receiver update the transmitter (BS) power P_(BS)^((n + 1)).

Alternatively, if no estimations and calculations are to be done by therelay stations, unprocessed results of the pilots are forwarded to acentralized functionality, in the receiver for example, and relevanttransmission parameters transmitted to each relay station.

Relay Stations Activation Control

The method and architecture of the present invention may advantageouslybe used for deciding which relay stations 215:k to include in acommunication, either at the establishment of the communication orduring the communication session. As some relays experiencing poor SNRconditions on either link (transmitter-relay and relay-receiver) orboth, they may contribute very little to the overall SNR improvements.Yet, those relays may still consume significant power due to receiver,transmitter and signal processing functions. It may also be of interestto have some control means to localize relay interference generation tofewer relays. Hence, it may therefore be considered to be wasteful touse some of the relay stations. Consequently, one desirable function isto activate relays based on predetermined criteria. Such criteria may bea preset lower threshold of acceptable SNR on either link, both links orthe contribution to the effective SNR. The limit may also be adaptableand controlled by some entity, preferably the receiving station as ithas information on momentary effective SNR. The relay may hence, e.g.together with power control information and cannel estimation symbols,receive a relay activation SNR threshold Γ_(Active) from the receiver towhich the expected SNR contribution is compared against, and ifexceeding the threshold, transmission is allowed, else not. The relayactivation SNR threshold Γ_(Active) corresponds to a common transmissionparameter, preferably determined by the receiver 220′ and distributed tothe relay station 215. The actual decision process, in which each relaystation uses local parameters (corresponding to the relativetransmission parameters) is distributed to the relay stations in themanner provided by the inventive method and architecture. This test,preferably performed in each relay prior to transmission, may e.g. beformulated according to: $\begin{matrix}{\frac{\Gamma_{{RS},k} \cdot \left( {\Gamma_{{RS},k} + 1} \right) \cdot \Gamma_{{MS},k}^{2}}{\varphi^{({n + 1})} \cdot \left( {\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1} \right)^{2}}\left\{ \begin{matrix}\left. {> \Gamma_{Active}}\Rightarrow{Transmit} \right. \\\left. {\leq \Gamma_{Active}}\Rightarrow{Silent} \right.\end{matrix} \right.} & (9)\end{matrix}$, but other conditions, depending on relay methods including alternativerelay diversity techniques, can also be used. For instance, the relayactivation condition may more generally be characterized as an objectivefunction ƒ₂ according to ƒ₂ (Γ_(RS,k),Γ_(MS,k)).

Moreover, The broadcasted message containing the Γ_(Active) couldfurther comprise fields that may be used to pinpoint specific relays(through assigned relay addresses) that should be incorporated, or isonly allowed to be used, or must excluded or any combination thereof.Other methods to address certain relays may e.g. be based on addressranges. This enables one to limit the number of involved relays asdesired.

From the above discussion and expression (9) it can be noted that thereceiver 220′ may, upon experiencing weakening SNR, for example due tothe movement of the MS, choose to order a increased transmission powerand/or to include more relay stations 215 by lowering the thresholdΓ_(Active). Other communication quality conditions, such as packet orbit error rate, may also be used by the receiver to trigger changes inthe common parameters, such as a joint transmit power sealing of allrelay powers.

Relay activation control may be incorporated in the power and phasecontrol algorithm described with reference to FIG. 6, by modifying thesteps 650-670, so that:

in 650: the receiver 220′ also determines an activation SNR thresholdΓ_(Active)

in 660: the receiver 220′ also broadcast Γ_(Active) to the relaystations 215:k.

in 670: each relay station 215:k firstly determines if to broadcastusing the activation SNR threshold Γ_(Active), for example according toexpression (9)

The method and architecture according the present invention may beadapted to other topologies than the above exemplified. The topology inFIG. 5 may, for example, be modified to include multiple antennas ineach relay station as shown in FIG. 7. The benefit in doing that is thatthe number of relay stations can be reduced while still getting similartotal antenna directivity gain. If each antenna element is separatedmore than the coherence distance, diversity gain is also provided. Inall, this can reduce the cost, while providing near identicalperformance. However, reducing the number of relays may have adetrimental impact due to shadowing (i.e. log normal fading) and must becarefully applied. From signal, processing and protocol point of view,each antenna can be treated as a separate relay station. Another benefitof this approach is however that internal and other resources and may beshared. Moreover, relaying may potentially be internally coordinatedamong the antennas, thereby mitigate interference generation towardsunintended receivers.

The communication quality may be further improved by also incorporatethe direct signal from the transmitter 210 to the receiver 220. Thereare at least two conceivable main methods to incorporate the signal fromthe transmitter. FIG. 8, depicts the topology when direct transmissionfrom the transmitter is also considered.

In the first method, two communication phases are required. The receivercombines the signal received directly from the transmitter, in the firstphase, with the relay transmission, from the second phase. This issomewhat similar to receiver based combining in the classical relaychannel, but with coherent combining based relaying. Maximum ratio orinterference rejection combining may be employed.

In the second method, Transmit-relay oriented Coherent Combining, onlyone communication phase is used, and used for coherent combining of thedirect signal from the transmitter to the receiver with the relaysignals. This can be made possible if relays can transmit and receiveconcurrently, e.g. over separated antennas. The phase of a_(k) must thenensure alignment of relayed signal with the direct signal asarg{a _(k)}=−arg{h _(1,k)}−arg{h _(2,k)}−arg{h _(BS,MS) }+c ₁, where h_(BS,MS) is the complex channel from the basestation to themobile station. A consequence of incorporating the direct signal forcoherent combining is that the relays must adaptively adjust their phaserelative the direct signal. A closed loop can be used for this. Similarto the normalization factor power control, the receiver issues phasecontrol messages to the whole group of relay stations, but with a deltaphase θ to subtract from the calculated phase compensation(−arg{h_(1,k)}−arg{h_(2,k)}).

As the basestation does not induce any noise through its transmission,its transmit power does not need to be adjusted for optimal performanceas was needed for the relays. Instead, performance increasesmonotonically with increasing basestation transmit power. One option ishowever to try to minimize the overall transmit power, aggregate relaypower and basestation power. The parameter setting for this is similarto what has been derived in the discussion on regenerative relaying,assuming that the basestation is considered as a relay. In addition toabove, multiple antenna elements at the transmitter may also be used,similar to the discussions on relays with multiple antennas.

The derivation of the relative and common transmission parameters isalso directly applicable to multi carrier transmission, such as OFDM byhandling each subcarrier independently. This will then include a commonamplitude normalization, phase and distributed relay amplitudecompensation per subcarrier. For doing this, the path overFFT-processing-IFFT is taken, or possible through time domain filtering.The power control may send a normalization factor φ and relay powerindication P_(RS) in vector form to optimize performance per subcarrier.A more practical solution, is to send φ and P_(RS) as scalars, acting onall subcarriers. In case of subcarrier optimization, the power controlmay then try to minimize power the total transmit power over allsubcarriers to meet desired communication quality. This then providessome diversity gain in the frequency domain.

Another OFDM aspect is that it is a preferred choice for thetransmit-relay oriented Coherent Combining described above. The reasonis that the cyclic prefix allow for some short relay transfer latency,where phase and amplitude is modified through a time domain filterenabling immediate transmission.

For single carrier transmissions, such as CDMA, and with frequencyselective channels, a frequency domain operation similar to OFDM may beemployed or optionally the phase alignment can be performed on thestrongest signal path, or with a time domain filter as discussed forOFDM.

For coherent combining to work, it is important to synchronize relaystation frequency to a common source. In a cellular system, the BS is anatural source as since the clock accuracy is generally better at thebasestation than in any mobile station. This function can exploit theregular frequency offset compensation as performed in traditional OFDMreceiver implementations, that mitigates inter channel interference.

However, the relays may optionally exploit GPS for frequencysynchronization, if available.

While the invention has primarily been described in a context ofcoherent combining, the invention is not limited hereto. The inventionmay be applied on various types of existing and forseen methods for2-hop (cooperative) relaying. In the most general case, the transmitparameters of the relays are functions of communication characteristicsof the first link, communication characteristics of the second link, ora combination thereof. The communication quality has been describedoutgoing from complex channel gain (suitable for coherent combining),however when other schemes are considered (offering diversity and/orspatial multiplex gains), other link characteristic metrics may be ofmore relevance. As an example, for Alamouti diversity it may be morepreferable to use average path gain metric, G, instead of complexchannel gains, h.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included with the spirit andscope of the appended claims.

Detailed Derivation

In the analysis we assume that there are K relay stations arbitrarilylocated. Each relay station kε{1, 2, . . . , K} receives a signalcomposed of an attenuated version of the desired signal, e.g. modeled ascomplex Gaussian x˜N(0,1), as well as a noise plus interference term,n_(RS,k), according toy _(k) =h _(1,k) −{square root}{square root over (P _(BS) )}· x+n_(RS,k), where h_(1,k) is the complex path gain from the basestation to relaystation k and P_(BS) is the transmit power of the basestation.

In the relay, y_(k) is (for analytical tractability) normalized to unitpower, and multiplied with a complex factor that generates output z_(k).subsequently z_(k) is sent over link two, towards the receiver and is onits way attenuated with complex path gain h_(2,k), where it issuper-positioned with signals from other relays and noise andinterference is added.

As it is assumed that each relay normalize the received power plus noiseto unit power prior amplification and phase adjustment, the relaytransmit power constraint can be incorporated in the analysis by lettingeach station k use transmit power$P_{k} = \frac{P_{RS} \cdot {a_{k}}^{2}}{\sum\limits_{k = 1}^{K}\quad{a_{k}}^{2}}$, where P_(RS) is the total transmit power of all relay stations, anda_(k) is a un-normalized complex gain factor for relay station k.

For aggregate power constrained relay transmission, the SNR at thereceiver (Mobile Station, MS, assumed here) may then be written$\Gamma = \frac{{{\sum\limits_{k = 1}^{k}\quad{\frac{\sqrt{P_{RS}} \cdot a_{k}}{\sqrt{\sum\limits_{q = 1}^{K}\quad{a_{q}}^{2}}} \cdot \frac{h_{1,k} \cdot \sqrt{P_{BS}}}{\sqrt{{{h_{1,k}}^{2}P_{BS}} + \sigma_{{RS},k}^{2}}} \cdot h_{2,k}}}}^{2}}{{\sum\limits_{k = 1}^{K}\quad{\frac{P_{RS} \cdot {a_{k}}^{2}}{\sum\limits_{q = 1}^{K}\quad{a_{q}}^{2}} \cdot \frac{\sigma_{{RS},k}^{2}}{{{h_{1,k}}^{2}P_{BS}} + \sigma_{{RS},k}^{2}} \cdot {h_{2,k}}^{2}}} + \sigma_{MS}^{2}}$, where σ² _(MS) is the noise plus interference level at the mobilestation.

A condition for coherent combining is phase alignment of signals, whichcan be achieved by ensuringarg{a _(k)}=−arg{h _(1,k)}−arg{h _(2,k) }+c ₁, where c₁ is an arbitrary constant

The expression for the effective SNR resulting from coherent combiningmay then be rewritten as$\Gamma_{Eff} = \frac{{{\sum\limits_{k = 1}^{K}\quad{{a_{k}}\frac{\sqrt{\Gamma_{{RS},k}} \cdot \sqrt{\Gamma_{{MS},k}}}{\sqrt{\Gamma_{{RS},k} + 1}}}}}^{2}}{\sum\limits_{k = 1}^{K}\quad{{a_{k}}^{2} \cdot \frac{\Gamma_{{MS},k} + \Gamma_{{RS},k} + 1}{\Gamma_{{RS},k} + 1}}}$, where$\Gamma_{{RS},k} = \frac{{h_{1,k}}^{2}P_{BS}}{\sigma_{{RS},k}^{2}}$, and$\Gamma_{{MS},k} = \frac{{h_{2,k}}^{2}P_{RS}}{\sigma_{{MS},k}^{2}}$Note that Γ_(MS,k) is a “virtual SNR” in the sense that it is the SNR ifrelay station^(k) would use all aggregate relay stations transmit powerby itself.

It is noticed that the SNR expression has the form$\Gamma_{Eff} = \frac{{{\sum\limits_{k = 1}^{K}{{a_{k}} \cdot c_{1,k}}}}^{2}}{\sum\limits_{k = 1}^{K}{{a_{k}}^{2} \cdot c_{2,k}}}$which can be transformed by using|b _(k)|² =|a _(k)|² ·c _(2,k), which yields$\Gamma_{Eff} = \frac{{{\sum\limits_{k = 1}^{K}{{b_{k}} \cdot \frac{c_{1,k}}{\sqrt{c_{2,k}}}}}}^{2}}{\sum\limits_{k = 1}^{K}{b_{k}}^{2}}$Now, the nominator is upper limited by Cauchy-Schwarz's inequality${{\sum\limits_{k = 1}^{K}{{b_{k}} \cdot \frac{c_{1,k}}{\sqrt{c_{2,k}}}}}}^{2} \leq {\sum\limits_{k = 1}^{K}{{b_{k}}^{2} \cdot {\sum\limits_{k = 1}^{K}{\frac{c_{1,k}}{\sqrt{c_{2,k}}}}^{2}}}}$, hence for an optimal b_(k) equality can be attained and the resultingSNR is then$\Gamma_{Eff}^{(\max)} = {\frac{{{\sum\limits_{k = 1}^{K}{{b_{k}} \cdot \frac{c_{1,k}}{\sqrt{c_{2,k}}}}}}^{2}}{\sum\limits_{k = 1}^{K}{b_{k}}^{2}} = \frac{\sum\limits_{k = 1}^{K}{{b_{k}}^{2} \cdot {\sum\limits_{k = 1}^{K}{\frac{c_{1,k}}{\sqrt{c_{2,k}}}}^{2}}}}{\sum\limits_{k = 1}^{K}{b_{k}}^{2}}}$

This may be conveniently expressed in SNRs as$\Gamma_{Eff}^{(\max)} = {\sum\limits_{k = 1}^{K}\frac{\Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1}}$

Through identification, it is seen that the maximum SNR can be attainedif ${b_{2}} = {{Const} \cdot \frac{c_{1,k}}{\sqrt{c_{2,k}}}}$, where Const is an arbitrary constant that can be set to one forconvenience.

From power control perspective, it is interesting to note that thenominator is exactly the square of the denominator for optimum SNR. Thisknowledge can therefore be used as a power control objective.

Using the reverse transformation, one yields${a_{k}} = \frac{c_{1,k}}{c_{2,k}}$, or expressed in SNRs${a_{k}} = \frac{\sqrt{\Gamma_{{RS},k}} \cdot \sqrt{\Gamma_{{MS},k}} \cdot \sqrt{\Gamma_{{RS},k} + 1}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1}$Hence a relay receiving a signal y_(k) can determine z_(k) bydetermining $\begin{matrix}{z_{k} = {\frac{\sqrt{P_{RS}}}{\sqrt{\sum\limits_{k = 1}^{K}{a_{k}}^{2}}} \cdot \frac{{{\mathbb{e}}^{{- j} \cdot {({{\arg{(h_{1,k})}} + {\arg{(h_{2,k})}}})}} \cdot \sqrt{\Gamma_{{RS},k}}}\sqrt{\Gamma_{{MS},k}}\sqrt{\Gamma_{{RS},k} + 1}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1} \cdot}} \\{\frac{y_{k}}{\sqrt{{{h_{1,k}}^{2}P_{BS}} + \sigma_{{RS},k}^{2}}}} \\{= {y_{k} \cdot \frac{1}{\sqrt{\sum\limits_{k = 1}^{K}{a_{k}}^{2}}} \cdot \frac{\sqrt{P_{BS} \cdot \Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}}{\sigma_{{RS},k} \cdot \left( {\Gamma_{{BS},k} + \Gamma_{{MS},k} + 1} \right)} \cdot {\mathbb{e}}^{{- j} \cdot {({{\arg{(h_{1,k})}} + {\arg{(h_{2,k})}}})}}}}\end{matrix}$Regenerative Relaying Add-on

If the SNR at a relay station is high enough, the received signal may bedecoded prior relaying the signal. To model this behavior, let's saythat larger than a minimum SNR, Γ_(Decode), is sufficient for decoding.The benefit in doing this, is that forwarding of detrimental noise (andinterference) can be avoided all together, and hence result in a furtherenhanced SNR at the receiver. In this case however, the decoded signalshould be phase compensated only for the second hop, i.e.arg{a _(k)}=−arg{h _(2,k)}By setting σ² _(KS,k)=0 for those stations in the previous expressions,one can derive the magnitude of the multiplicative factor |a_(k)| aswell as the contribution to the SNR improvement. The combination of bothnoise-free (regenerative) and noisy (non-regenerative) transmission thentakes the form$\Gamma_{Eff}^{(\max)} = {\sum\limits_{k = 1}^{K}\left\{ \begin{matrix}{\frac{\Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1},} & {{{if}\quad\Gamma_{{RS},k}} < \Gamma_{Decode}} \\{\Gamma_{{MS},k},} & {{{if}\quad\Gamma_{{RS},k}} \geq \Gamma_{Decode}}\end{matrix} \right.}$, and ${a_{k}} = \left\{ {\begin{matrix}{\frac{\sqrt{V_{{RS},k}} \cdot \sqrt{\Gamma_{{MS},k}} \cdot \sqrt{\Gamma_{{RS},k} + 1}}{\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1},} & {{\text{if}\quad\Gamma_{{RS},k}} < \Gamma_{Decode}} \\{\sqrt{\Gamma_{{MS},k}},} & {{{if}\quad V_{{RS},k}} \geq \Gamma_{Decode}}\end{matrix},} \right.$, and ${\arg\left\{ a_{k} \right\}} = \left\{ \begin{matrix}{{{{- \arg}\left\{ h_{l,k} \right\}} - {\arg\left\{ h_{2,k} \right\}}},} & {{{if}\quad\Gamma_{{RS},k}M} < \Gamma_{Decode}} \\{{{- \arg}\left\{ h_{l,k} \right\}},} & {{{if}\quad\Gamma_{{RS},k}} \geq \Gamma_{Decode}}\end{matrix} \right.$Note that Γ_(RS,k)<Γ_(Decode) is only a model useful to assessperformance in a mixed non-regenerative and regenerative relayingscenario. In practice, the upper expressions, i.e. corresponding toΓ_(RS,k)<Γ_(Decode), are used when the signal is not forwarded in anon-regenerative manner, and the lower expressions, i.e. correspondingto Γ_(RS,k)>Γ_(Decode), are used when the signal is not forwarded in aregenerative manner.

References

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1. A method of performing communication in a two-hop wirelesscommunication network, wherein a transmitter, a receiver and at leastone relay station are engaged in a communication session, and the relaystation forwards signals from a first link between the transmitter andthe relay station to a second link between the relay stations and thereceiver, wherein the forwarding performed by the at least one relaystation is adapted as a response to estimated radio channelcharacteristics of at least the first link.
 2. The method according toclaim 1, wherein the forwarding performed by the at least one relaystation is adapted as a response to estimated radio channelcharacteristics of both the first and second link.
 3. The methodaccording to claim 1, wherein the communication session involves aplurality of relay stations and their respective forwarding is adaptedbased on a relative transmission parameter which is specific for eachrelay station and a common transmission parameter which is common to allrelay stations.
 4. The method according to claim 1, wherein the methodcomprises the steps of: characterizing the radio paths of the first andsecond link by the use of pilots; determine at least one relativetransmission parameter at least partly based on both of the channelestimates of each relay stations paths of the first and second link;determine at least one common transmission parameter based; distributingat least said common transmission parameter to all relay station;forwarding the signal from the first link on the second link, whereinthe forwarded signal is adapted based on each relay stations relativetransmission parameter and the common transmission parameter.
 5. Themethod according to claim 1, wherein the adaptation of the transmittedsignal comprises an adjustment of phase.
 6. The method according toclaim 1, wherein the adaptation of the transmitted signal comprises anadjustment of transmission power.
 7. The method according to claim 1,wherein the adaptation of the transmitted signal comprises an adjustmentof transmission power and phase.
 8. The method according to claim 1,wherein the adaptation of the transmitted signal comprises an adjustmentof parameters relating to diversity.
 9. The method according to ofclaims 8, wherein the adaptation of the transmitted signal comprises anadjustment of parameters relating to delay diversity.
 10. The methodaccording to of claims 8, wherein the adaptation of the transmittedsignal comprises an adjustment of parameters relating to space timecoded diversity.
 11. The method according to claim 1, wherein the stepof using the relay station's respective relative transmission parameterand the common transmission parameter(s) to adapt the subsequenttransmissions on link 2, comprises to, on the reception of signal y_(k),transmit the signal:$z_{k} = {y_{k} \cdot \frac{1}{\varphi} \cdot \frac{\sqrt{P_{RS} \cdot \Gamma_{{RS},k} \cdot \Gamma_{{MS},k}}}{\sigma_{{RS},k} \cdot \left( {\Gamma_{{RS},k} + \Gamma_{{MS},k} + 1} \right)} \cdot {\mathbb{e}}^{{- j} \cdot {({{\arg{(h_{1,k})}} + {\arg{(h_{2,k})}}})}}}$wherein the parameters Γ_(RS,k) and Γ_(MS,k) are the locally determinedrelative transmission parameters based on the channel estimates h_(1,k)and h_(2,k), P_(BS) is the transmit power of the transmitter, σ² _(RS)is the noise and interference level at the relay station, P_(RS) is theaggregated transmit power from all relay stations, σ² _(MS) is the noiselevel at each receiver, and wherein the normalizing factor φ is a commonparameter based on the total communication quality experienced by thereceiver.
 12. A relay station adapted for use in a two-hop wirelesscommunication network, wherein the network comprises a transmitter, areceiver and at least one relay station, wherein the relay station isadapted to forwarding signals from a first link between the transmitterand the relay station to a second link between the relay stations andthe receiver wherein the relay station is provided with means foradapting the forwarding based on a characterization of at least thefirst link.
 13. The relay station according to claim 14, wherein therelay station adapt the forwarding as a response to estimated radiochannel characteristics of both the first and second link.
 14. The relaystation according to claim 14, wherein the relay station is furtherprovided with means for performing channel characterization and meansfor determining relative transmission parameters based on the channelcharacterization, and the forwarding is at least partly based on saidrelative transmission parameters.
 15. The relay station according toclaim 15, wherein the relay station is further provided with means forreceiving a common transmission parameter, and the forwarding is atleast partly based on said relative transmission parameters and saidcommon transmission parameter
 16. A system adapted for communication ina two-hop wireless communication network, wherein the network comprisesa transmitter, a receiver and at least one relay station, wherein therelay station is adapted to forwarding signals from a first link betweenthe transmitter and the relay station to a second link between the relaystations and the receiver wherein the relay station usescharacterization of at least the first link for the forwarding on thesecond link.
 17. The system according to claim 16, wherein the relaystation adapt the forwarding as a response to estimated radio channelcharacteristics of both the first and second link.
 18. The systemaccording to claim 18, wherein the relay station is further providedwith means for performing channel characterization and means fordetermining relative transmission parameters based on channelcharacterization and the forwarding is at least partly based on saidrelative transmission parameters.
 19. The system according to claim 17,wherein the system is provided with means for determining a commontransmission parameter which is based on the total communication qualitybetween the transmitter and the receiver, and the relay station isfurther provided with means for receiving the common transmissionparameter and the forwarding on the second link is at least partly basedon said relative transmission parameters and said common transmissionparameter.
 20. A receiver adapted for use in a two-hop wirelesscommunication network, wherein the network comprises a transmitter, thereceiver and at least one relay station, wherein the relay station isadapted to forwarding signals from a first link between the transmitterand the relay station to a second link between the relay stations andthe receiver, wherein the receiver is provided with means fordetermining at least one relative transmission parameter which is basedon a characterization of at least the first link, and means fordistributing said relative transmission parameter to the relay station.21. The receiver according to claim 20, wherein the determining meansare adapted to determine a plurality of relative transmission parameter,one for each relay station which are engaged in the communicationsession.
 22. The receiver according to claim 20, wherein the relativetransmission parameter is based on characterisations of both the firstand second link.
 23. The receiver according to claim 20, wherein thereceiver i further provided with means for determining a commontransmission parameter which is based on the total communication qualitybetween the transmitter and the receiver.
 24. A base station adapted foruse in a two-hop wireless communication network, comprising a receiveraccording to claim
 20. 25. A mobile station adapted for use in a two-hopwireless communication network, comprising a receiver according to claim20.